Ethylene type polymers such as an ethylene polymer and an ethylene/.alpha.-olefin copolymer have been conventionally molded by various molding (or forming) methods such as blow molding, vacuum or pressure forming, calendering, inflation molding, extrusion molding, expansion molding, stretched film molding and injection molding, and they have been widely employed.
The ethylene type polymers are required to have various properties depending on the purpose or the molding method. For example, in the production of films by means of high-speed inflation molding, ethylene type polymers having high melt tension should be employed to inhibit instability or breakage of bubbles. In the production of cans for industrial chemicals, drum cans, bottles or the like by means of blow molding, ethylene type polymers having high swell ratio (diametrical swell ratio) should be employed to obtain blow molded articles having good shape of pinch-off portions and narrow wall thickness distribution. The blow molded articles are generally required to have properties of high impact strength, and in addition thereto they are recently required to have high rigidity to improve durability and economical effects.
By the way, the ethylene type polymers are conventionally prepared using catalysts such as Ti catalysts or Cr catalysts (Philips catalysts) or by a high-pressure method. Among various ethylene type polymers, those prepared by the use of Ziegler-Natta catalysts typified by Ti catalysts, esp. MgCl.sub.2 supported type Ti catalysts, have molecular structure almost free from long-chain branches and show excellent rigidity and impact resistance. However, they are inferior in the moldability to the ethylene type polymer prepared by the use of Cr catalysts. Meanwhile, the ethylene type polymers prepared by the high-pressure method and by the use of Cr catalysts have higher melt tension and swell ratio than those by the use of Ziegler-Natta catalysts and have excellent moldability. However, they have molecular structure with long-chain branches and are inferior in the rigidity and impact resistance.
In order to improve the properties of the ethylene type polymers, various proposals have been made. For example, in Japanese Patent Laid-Open Publication No. 12735/1980, a blend of polyethylene prepared by the use of Ziegler-Natta catalyst and polyethylene prepared by a high-pressure method is proposed. In Japanese Patent Laid-Open Publication No. 36546/1985, a blend of polyethylene prepared by the use of Ziegler-Natta catalyst and polyethylene prepared by the use of Cr catalyst is proposed. However, these polyethylene blends are inferior to the ethylene type polymers prepared by the use of Ziegler-Natta catalysts in rigidity and impact resistance, though they are improved in the moldability.
Under such circumstances as mentioned above, the present inventors have studied, and they have found that an ethylene type polymer composition comprising a specific high-density ethylene type polymer and a specific ethylene type polymer, in which at least one of them is prepared by the use of a metallocene catalyst and the density ratio therebetween exceeds 1, is excellent in moldability, stress crack resistance, mechanical strength and rigidity. The present inventors have also found that such a composition can be molded into articles for a wider variety of uses by various methods such as blow molding, vacuum or pressure forming, calendering, inflation molding, extrusion molding, expansion molding, stretched film molding and injection molding. Based on the finding, the present invention has been accomplished.
The present invention has an object to provide an ethylene type polymer composition which is capable of being molded into articles for a wider variety of uses by various methods such as blow molding, vacuum or pressure forming, calendering, inflation molding, extrusion molding, expansion molding, stretched film molding and injection molding and from which molded articles of excellent mechanical strength and rigidity are obtainable.